In recent years, fluorescent tubes used as lighting devices or backlight units for liquid crystal display (LCD) panels have been replaced with other types of light emitting elements such as light emitting diodes (LEDs), which have improved properties in energy saving, life span, control, and the like.
FIG. 1 is a circuit diagram illustrating a configuration example of a conventional light emitting device. A light emitting device 1r includes a plurality of LED strings 4_1 to 4_N (which may also be referred to as “LED bars”) of multiple N channels CH_1 to CH_N, and a driving circuit 2r. 
Each LED string 4 includes a plurality of LEDs which are connected to each other in series. The driving circuit 2r includes a DC-DC converter 10, a plurality of current drivers 20_1 to 20_N, and a controller 30. For example, the current drivers 20_1 to 20_N and the controller 30 may be integrated into a single IC (Integrated Circuit) chip or module.
The DC-DC converter 10 supplies a drive voltage VOUT to anodes of the LED strings 4_1 to 4_N by boosting an input voltage VIN. The DC-DC converter 10 is a step-up switch power supply that includes a switch transistor M1, an inductor L1, a rectifier diode D1, and an output capacitor C1.
The current drivers 20_1 to 20_N are configured in the channels CH_1 to CH_N, respectively, and given i (1≦i≦N), the current driver 20—i is connected to a cathode of the corresponding LED string 4—i. The current driver 20—i is, thus, configured to supply a drive current ILED—i to the corresponding LED string 4—i. 
The controller 30 controls the DC-DC converter 10. Specifically, the controller 30 performs a switch operation on the switch transistor M1 in the DC-DC converter 10 so as to adjust a lowest voltage from among cathode voltages VLED—1 to VLED—N of the LED strings 4_1 to 4_N in the channels CH_1 to CH_N to be equal to a predetermined reference voltage VREF. The reference voltage VREF is set as a voltage level at which the drive current ILED can be generated without saturating an internal transistor of the current driver 20.
For example, the controller 30 may include an error amplifier 32, a pulse modulator 34, a driver 36, an OVP (Over-Voltage Protection) comparator 38, and a logic part 40.
The error amplifier 32 generates an error voltage VFB by amplifying an error (e.g., a gap in voltage) between the reference voltage VREF and the lowest voltage from among the cathode voltages VLED—1 to VLED—N in the channels CH_1 to CH_N. The pulse modulator 34 generates a pulse signal SPM that has a duty cycle according to the error voltage VFB. For example, the pulse modulator 34 adjusts the duty cycle of the pulse signal SPM by performing pulse width modulation or pulse frequency modulation.
Based on the pulse signal SPM, the driver 36 performs a switch operation on the switch transistor M1 of the DC-DC converter 10. For example, it may be assumed that a voltage drop VF—i across an LED string 4—i in an i-th one of the channels CH_1 to CH_N is the largest. Then, the cathode voltage VLED—i of the LED string 4—i in the i-th channel is lowest and a feedback is applied to adjust the cathode voltage VLED—i to be equal to the reference voltage VREF. Accordingly, the drive voltage VOUT is stabilized to a target level according to following Equation (1):VOUT=VF—i+VREF  Equation (1)
A pair of resistors R11 and R12 divides the drive voltage VOUT. The OVP comparator 38 compares the divided drive voltage VOUT′ with a predetermined threshold voltage VOVP2, and asserts an OVP signal (for example, generates a high level signal) when VOUT′>VOVP2. When the OVP signal is asserted, the logic part 40 is configured to perform a predetermined over-voltage protection process. For example, when the OVP signal is asserted, the logic part 40 may stop the current drivers 20_1 to 20_N and the switch operation of the DC-DC converter 10.
The configurations of the driving circuit 2r are described in the above. The driving circuit 2r is capable of stabilizing the drive voltage VOUT to a lowest level within a range where the LED string 4 of each channel can be illuminated with desired brightness. As such, it is possible to drive the LED strings 4_1 to 4_N with high efficiency.
In addition, as the OVP comparator 38 detects an overvoltage, it is possible to prevent the drive voltage VOUT from exceeding a threshold voltage VOVP1 (=VOVP2×(R11+R12)/R12) and protect the DC-DC converter 10, the LED strings 4, and various other components in the current driver 20 against the overvoltage.
However, the light emitting device 1r of FIG. 1 may involve the following problems.
If the light emitting device 1r is not completely sealed, foreign substances such as fine dusts and metal pieces may be introduced into the light emitting device 1r. Due to such foreign substances, any nodes may be short-circuited to a voltage potential of ground (which may be referred to as “ground fault”) or short-circuited to a voltage potential of a power supply (which may be referred to as “power source fault”). Such a ground fault or a power source fault may make a feedback control ineffective and cause a problem in which circuit elements may be heated by a large current flowing through any path.
For example, it may be assumed that an LED terminal LED_N in an N-th channel is in a state of a ground-fault due to a path 6, which is indicated by dashed lines having dots. FIG. 2 illustrates diagrams of operation waveforms of the light emitting device 1r illustrated in FIG. 1 under a state of a ground fault.
Before time t0, the light emitting device 1r is in a normal state, a cathode voltage (i.e., first detection voltage) VLED—N of the LED string 4_N in the N-th channel is stabilized to the reference voltage VREF, and a drive current ILED—NORM flows through the LED string 4_N. The drive voltage VOUT is stabilized to a voltage level according to the following Equation (2):VOUT—NORM=VF—NORM+VREF  Equation (2)In Equation (2), VF—NORM indicates a forward voltage (i.e., a voltage drop) of the LED string 4_N when the drive current ILED—NORM flows.
If a ground fault occurs due to the path 6 at time t0, the cathode voltage VLED—N decreases to near 0V. The controller 30 then increases the drive voltage VOUT in order to increase the cathode voltage VLED—N to the reference voltage VREF. However, since the cathode voltage VLED—N is not increased in spite of increasing the drive voltage VOUT, a feedback is applied to further increase the drive voltage VOUT.
In this case, since the drive voltage VOUT is directly applied between an anode and a cathode of the LED string 4_N in the N-th channel, the drive current ILED—N cannot be controlled, which results in increasing the drive current ILED—N. This state lasts until time t1 at which the drive voltage VOUT reaches the OVP threshold voltage VOVP1. After time t1, as the OVP is activated, the drive voltage VOUT begins to decrease and thus the drive current ILED—N decreases.
As such, in the light emitting device 1r of FIG. 1, the flow of a large current through the LED string 4_N is continued between times t0 and t1.